Hollow-Sphere Tin Nanocatalysts for Converting CO2 into Formate

ABSTRACT

Three-dimensional (3D) hollow nanosphere electrocatalysts that convert CO2 into formate with high current density and Faradaic efficiency (FE). The SnO2 nanospheres were constructed from small, interconnected SnO2 nanocrystals. The size of the constituent SnO2 nanocrystals was controlled between 2-10 nm by varying the calcination temperature and observed a clear correlation between nanocrystal size and formate production. In situ Raman and time-dependent X-ray diffraction measurements confirmed that SnO2 nanocrystals were reduced to metallic Sn and resisted microparticle agglomeration during CO2 reduction. The nanosphere catalysts outperformed comparably sized, non-structured SnO2 nanoparticles and commercially-available SnO2 with a heterogeneous size distribution.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 63/147,481 filed 9 Feb. 2021.

GOVERNMENT RIGHTS CLAUSE

This invention was made with Government support under contract89243318CFE000003 awarded by the U.S. Department of Energy. TheGovernment has certain rights in this invention.

INTRODUCTION

Rationally controlling electrocatalyst structure from the atomic tomicron scales is important for designing new materials that canelectrochemically convert CO₂ into value-added chemicals and fuels.¹⁻⁶The CO₂ reduction reaction (CO₂RR) has a rich structure-sensitivity, andsubstantial efforts have been devoted to improving performance bycontrolling the catalyst size, morphology, composition, crystallographicorientation, and surface structure.¹⁻⁶ Recent work has introducedthree-dimensionality into CO₂RR electrocatalyst design by assemblingsuperstructures from nanoscale building blocks, including architecturessuch as micro/nano-spheres, flowers and dendrites, porous foams, inverseopals, and others.⁷⁻²⁰ These 3D structures offer high surface area,large density of active sites, and better accessibility for reactantsand intermediates that can accelerate CO₂RR kinetics and improve productselectivity and catalyst stability.^(7-12,14-16,19)

Formic acid (HCOOH), electrochemically produced as formate (HCOO⁻), is aCO₂RR product with wide agricultural, industrial, chemical andpharmaceutical uses.²¹⁻²⁴ Formic acid/formate has also been identifiedas an emerging fuel for fuel cells,^(25,26) a liquid hydrogen carrierwith high volumetric capacity (53 g of H₂ per liter),^(27,28) and forbiomass upgrading applications.²⁹ Industrial formic acid production fromfossil fuel precursors is extremely carbon intensive,²² butelectrochemically converting CO₂ to formate, followed by down-streamelectrodialysis purification into formic acid,³⁰ could provide a carbonneutral or carbon negative route for producing this versatile chemical.

Sn-based materials are some of the most effective CO₂RR electrocatalystsfor formic acid/formate production.^(5,6,8-13,16,18) However, theperformance of most Sn-based catalysts is still inadequate for practicalapplications because of low current densities (typically 10˜25 mAcm_(geo) ⁻² in aqueous H-cells; Table 1), high overpotentials, and poorlong-term stability^(5,6,8-13,16,18,31-38) Therefore, further catalystdesign efforts is required to boost CO₂RR activity, improve efficiency,and validate operation at high current density in realistic devicearchitectures.

Two examples of tin oxide spheres for electrochemical CO₂ reduction toformic or formate have been reported in the literature by a researchgroup from China. The article “Novel hierarchical SnO₂ microspherecatalyst coated on gas diffusion electrode for enhancing energyefficiency of CO₂ reduction to formate fuel;” Applied Energy, 2016, 175,536-544. This article reported the synthesis of 1-3 μm large, dense SnO₂microspheres composed of 20-40 nm nanoparticles by hydrothermalself-assembled process at 180° C. for 24 h using SnCl₄ and D-glucosemonohydrate precursors. The size and morphology of hierarchical SnO₂microspheres were controlled by varying the volume ratio of ethanol todistilled water. These SnO₂ microspheres exhibited a maximum 62% formateFaradaic efficiency (FE) at −1.7 V vs. SHE (Standard HydrogenElectrode). Similarly, “Electrochemical CO₂ reduction to formic acid oncrystalline SnO₂ nanosphere catalyst with high selectivity andstability” Chinese J. Catal., 2016, 37, 1081-1088 reported a mixture ofSnO₂ nanoparticles and 500 nm˜1 μm nanosphere aggregates composed of20-25 nm nanoparticles that produced a maximum of 56% formate FE in 0.5M KHCO₃ and max 428 mg/L formate production rate in 0.7 M KHCO₃ at −0.56V vs. SHE. As described below, we have produced superior catalysts andsystems.

SUMMARY OF INVENTION

Our catalyst directly converts CO₂ and water into formate in anelectrochemical reactor, eliminating the need for the carbon intensivemethanol precursor. The catalysts can have activity at least about sixtimes higher than commercially-available SnO₂ catalysts, and higher thanthe best materials reported in the open scientific literature. Theimprovement in catalytic rates, efficiencies, and selectivity addresscore technical issues that have prevented the development of effectiveelectrocatalytic technologies for CO₂ utilization.

This invention provides the synthesis and application of a newnanostructured tin-oxide (SnO₂) nano catalysts that efficiently convertsCO₂ into formate with very high activity.

Our synthetic procedure produces a catalyst structure composed of ahollow-sphere constructed from interconnected SnO₂ nanoparticles (nps).The synthetic preparation allows us to tune the size of the constituentnanoparticles and control overall activity.

SnO₂ nanosphere electrocatalysts can be constructed from small,interconnected SnO₂ nanocrystals. Tuning thermal annealing temperaturesincreased formate production by controlling the crystallinity andparticle size of the constituent SnO₂ nanocrystals. SnO₂ nanospheresdemonstrated high Faradaic efficiencies, selectivities, and superiorcurrent densities toward formate production over a wide potential rangeduring H-cell testing. SnO₂ nanospheres surpassed non-templated SnO₂ npsof similar size and commercially-available SnO₂ catalysts, and exhibitedgood durability over 36 hours with intermittent cycles of operation. Theimproved CO₂-to-formate performance of SnO₂ nanospheres can beattributed to 3D structure with large electrochemical surface area andbetter resistance to particle sintering during CO₂RR.

In one aspect, the invention provides a SnO₂ powder, comprising at least90 mass % hollow spheres in the (diameter) size range of 175 to 225 nm;and wherein the hollow spheres are comprised of SnO₂. In some preferredembodiments, at least 90 mass % hollow spheres in the (diameter) are inthe size range of 180 to 220 nm, in some embodiments 190 to 210 nm. Inanother aspect, the invention provides a SnO₂ powder, comprising hollowspheres having a diameter of 100 nm or greater, wherein the spheres arecomprised of SnO₂ particles, and wherein at least 90 mass % of thespheres have diameters in a 10 nm range (for example from 200 to 220nm), or in a 7 nm range or a 5 nm range or a 3 nm range.

Any of the inventive aspects may further be characterized by one or anycombination of the following characteristics: the hollow spheres mayhave a wall thickness in the range of 20 to 35 nm or 25 to 30 nm; thehollow spheres may be comprised of nanocrystals having a mass averagediameter in the range of 5 to 15 nm, or 5-10 nm, or 6 to 9 nm; anaverage crystallite size, as measured by XRD, in the range of 5 to 10nm, or 6 to 9 nm, or 6 to 8 nm; the SnO₂ powder characterizable by adurability of maintaining a j_(formate) (mA cm_(geo) ⁻²) of at least 35or at least 40 or in the range of 40 to 55 at 1.2 V vs. RHE (any of theelectrochemical properties can be present over a period of 1 or 2 or 3or 4 days without regeneration, as measured according to theElectrochemical CO₂ Reduction Measurement that is described in theExamples section); the SnO₂ powder characterizable by a j_(total) (mAcm⁻² _(geo)) of at least 50 or at least 55 or in the range of 50 to 75at 1.2 V vs. RHE; the SnO₂ powder characterizable as having adouble-layer capacitance (mF cm⁻²) of at least 10 or 10 to 20 or 12 to15; the SnO₂ particles or an electrode comprising the SnO₂ particlescharacterized by an ESCA of at least 35 or at least 40 or at least 45,or in the range of 35 to 60 or 40 to 55 or 45 to 52 cm⁻²; and/or whereinthe particles, electrodes, or methods are characterizable by propertieswithin ±10% or ±20% or ±30% of the data shown in the Examples. Theinvention also includes methods of converting CO₂ to formate or formicacid comprising contacting an SnO₂-coated electrode with CO₂ and H₂O andpassing an electrical current through the electrode; wherein the CO₂ andH₂O react over the catalyst to form formate; and wherein the SnO₂comprises any of the compositions described herein.

In another aspect, the invention comprises a method of making a SnO₂catalyst, comprising: providing a suspension of polymer particles,combining a tin salt with the suspension, removing the liquid from thesuspension (preferably by evaporation) to form tin-coated polymerparticles, drying the tin-coated polymer particles, and calcining thedried particles to burn out the polymer particles leaving hollow SnO₂spheres. In some preferred embodiments, the method can be furthercharacterized by one or any combination of the following optionalfeatures: the suspension is an aqueous suspension; the polymer particlescomprising poly (methyl methacrylate) spheres, polystyrene spheres,carboxylic polystyrene spheres, poly(n-butyl acrylate-acrylic acid)spheres, carbon spheres, silica spheres, or other suitable sphericalparticles; calcining is preferably carried out at a temperature in therange of 300 to 600° C., or 400 to 575° C., or 450 to 550° C.

In a further aspect, the invention provides a catalyst ink comprisingSnO₂ particles dispersed in a liquid phase along with conductiveparticles and binder particles. Preferred compositions of ink compriseat least 50% or at least 70% or at least 80% SnO₂ particles. Preferredconductive particles comprise carbon black, carbon fibers, carbon orgraphene sheets, or carbon nanotubes; preferably the ink comprises atleast 2% or at least 5%, or in the range of 2% to 20%, or 3% to 15%conductive particles. Preferred binders are polymeric binders,preferably a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer such as Nation®. Preferably the ink comprises atleast 5% or at least 10%, or in the range of 5% to 40% binder. Theliquid phase preferably is primarily an alcohol or mixture of alcoholssuch as methanol, ethanol, isopropanol or n-propanol.

In another aspect, the invention provides an electrode, comprising aconductive substrate coated with the SnO₂ powder. Preferably, theconductive substrate comprises carbon, preferably a porous carbon paper.The invention also includes methods of making an electrode byimpregnating, drop-casting or coating an ink into or on a conductivesubstate. The invention also includes a system comprising an electrodecomprising a tin catalyst disposed in a solution that is saturated withCO₂, and further wherein the system or catalyst is characterizable by adurability of maintaining a j_(formate) (mA cm_(geo) ⁻²) of at least 35or at least 40 or in the range of 40 to 55 at 1.2 V vs. RHE for at leastone or at least two or at least three days or from one to five days. Theinvention also includes systems comprising the electrode disposed in asolution (preferably an aqueous solution) that is saturated with CO₂.Preferably, a circuit is formed with an anode wherein the anode andSnO₂-coated electrode are present in an electrochemical cell separatedby a proton exchange membrane.

In a further aspect, the invention provides a method of converting CO₂to formate or formic acid comprising contacting an SnO₂-coated electrodewith CO₂ and H₂O and passing an electrical current through theelectrode; wherein the CO₂ and H₂O react over the catalyst to formformate. The electrode has a Faradaic efficiency to formate of at least50%, or at least 60%, or at least 70% or in the range of 60 to 85%;preferably conducted at a potential in the range of 0.7 to 1.4 V vs.RHE, or 0.9 to 1.3 V vs. RHE. During the reaction, the SnO₂ may beconverted to partly reduced (less than two oxygens per Sn) or metallicSn. The method/system preferably can be conducted for at least 24 hourswithout replacing or regenerating the electrode while maintainingFaradaic efficiency at the claimed level. The method/system may beconducted with a HCOO⁻ current density of at least 40, or at least 45,or in the range of 40 to 60 or 45 to 55 mA cm⁻² _(geo). Preferably, theSnO₂ particles have one or more of the characteristics of the SnO₂spheres described herein.

Advantageous features of the invention include formate production ratesat least two times or at least four times or at least six-fold higherthan commercially-available SnO₂ powders or SnO₂ powders prepared by thesame procedure as the templated particles but without the sphericalpolymeric templates; the ability to provide catalysis without preciousmetals; for example, less or equal to 1 mass % or 0.5 mass % of allprecious metals such as Au, Ag, Pt, Pd; unique synthetic techniqueproduce hollow sphere-like catalysts composed of small nanostructuredparticles that boost performance.

Throughout these descriptions, % refers to mass % unless indicatedotherwise. The electrochemical characteristics of the powders,electrodes and/or systems are measured as set forth in the Examples,specifically the Electrochemical CO₂ Reduction Measurement. Note thatthe term “characterizable by” means that the composition or system canbe measured to possess the property, like any other characteristic, theproperty can be latent until measured. Various aspects of the inventionare described using the term “comprising;” however, in narrowerembodiments, the invention may alternatively be described using theterms “consisting essentially of” or, more narrowly, “consisting of.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. (A) Scheme illustrating the synthesis of 3D hollow SnO₂nanospheres by a combined sol-gel and templating method. (B)Representative FE-SEM and (C) HR-TEM images of SnO₂ spheres calcined at500° C. (D) XRD crystallite size as a function of calcinationtemperature.

FIG. 2. (A) Formate partial current density vs. cathodic potential ofhollow SnO₂ nanospheres calcined at different temperatures. (B)Representative Faradaic efficiency for formate, CO, and H₂ for SnO₂nanospheres calcined at 500° C. (C) Geometric and (D) ECSA-normalizedformate current densities for commercial-SnO₂ nps, non-templated SnO₂nps (bottom trace), and the inventive SnO₂ nanospheres calcined at 500°C.

FIG. 3. (A) Long-term durability performance of the inventive SnO₂nanospheres (upper data points), non-templated SnO₂ nps, andcommercially-available SnO₂ nps at −1.2 V vs. RHE during several on/offcycles. (B) FE-SEM images of three electrodes before and after long-termelectrolysis. (C) Synchrotron-based XRD patterns of the best-in-classSnO₂ nanospheres collected after operating for various time periods at−1.2 V vs. RHE.

FIG. 4. FE-SEM images of SnO₂ nanospheres calcined at (A, B) 300° C.,(C, D) 400° C. and (E, F) 600° C. Higher thermal annealing temperaturesresulted in broken spherical shells.

FIG. 5. HR-TEM images of SnO₂ nanospheres calcined at 300° C., revealingthe spherical shells were ca. 5 nm-thick and composed of 2-3.5 nm SnO₂nanocrystallites. Inset of (A) is the corresponding FFT diffractionpattern showing polycrystalline tetragonal rutile SnO₂.

FIG. 6. HR-TEM images of SnO₂ nanospheres annealed at 500° C.,indicating thicker wall of 25-30 nm containing interconnected 6-9 nmSnO₂ nanocrystals with distinct grain boundaries. The TEM-determinednanocrystal diameter of 6-9 nm is consistent with the average 7 nmcrystallite size determined from X-Ray diffraction.

FIG. 7. Comparison of Faradaic efficiency for (A) formate, (B) CO, and(C) H₂ vs. potentials for SnO₂ nanospheres calcined from 300° C. to 600°C.

FIG. 8. Total geometric current densities for 3D SnO₂ nanosphere series.The bottom trace is from the nanospheres calcined at 300° C.

FIG. 9. (A) Formate selectivity (referring to total CO₂RR products) and(B) production rate of formate for SnO₂ nanospheres calcined at 500° C.

FIG. 10. (A) XRD patterns and (B) Raman spectra of non-templated SnO₂nps and com-SnO₂ nps compared with SnO₂ nanospheres calcined at 500° C.

FIG. 11. Comparison of Faradaic efficiency for (A) formate (inventive istop trace), (B) CO, and (C) H₂ vs. potentials of the best performingSnO₂ nanospheres, non-templated SnO₂ nps, and com-SnO₂ nps.

FIG. 12. Double-layer capacitance measurement in CO₂-purged 0.1 M KHCO₃electrolyte: (A-C) Cyclic voltammetry profiles measured in thenon-Faradaic region with different scan rates. (D) Scan rate dependenceof the current density for com-SnO₂ nps, non-templated SnO₂ nps and theinventive SnO₂ nanospheres electrodes.

FIG. 13. Core-level (A) Sn 4p+Pt 4f, (B) Sn 4s+Pb 4f, (C) Sn MNN+Zn 2p,and (D) Sn 3p+Fe 2p spectra of SnO₂ nanospheres electrode before andafter long-term stability run. No distinguishable features of trace Pb,Zn, Fe, or crossover Pt elements indicates these potential tracecontaminants were not deposited onto the catalyst surface during the36-hour electrolysis. A similar lack of contaminants was observed fornon-templated and commercially available SnO₂ after long-termelectrolysis.

FIG. 14. XRD crystallize size of starting SnO₂ and reduced β-Sn as afunction of time obtained from time-resolved XRD patterns of SnO₂nanospheres collected at −1.2 V vs. RHE.

DETAILED DESCRIPTION OF THE INVENTION

Catalysts were synthesized using a tin-salt precursor dissolved inalcohol and citric acid. A polymer template was mixed with the startingcatalyst precursor, dried in air and calcined at high temperatures toform the catalyst structures. We could control the resulting catalyststructure based on the synthetic conditions and calcination temperature.A preferred catalyst prepared at 500° C. comprises approximately 205-210nm diameter and 25-30 nm wall thickness hollow spheres constructed frominterconnected, about 10 nm SnO₂ nanoparticles. X-ray photoelectronspectroscopy confirmed the composition and oxidation state of the metal,and X-ray diffraction confirmed the nanocrystallite SnO₂ size of ˜7.5nm.

3D SnO₂ nanospheres were prepared by a combined sol-gel and templatingapproach (FIG. 1A). Negatively charged tin (II) citrate complex wasabsorbed on the surface of positively-charged poly(methyl methacrylate)(PMMA) spheres (diameter of ca. 220 nm) through electrostaticinteraction. The system underwent hydrolysis, condensation, nucleation,and self-assembly to create tin-containing coating layers on the surfaceof the PMMA spheres. Subsequent calcination in air between 300 to 600°C. converted these coating layers into SnO₂ nanocrystals and removed thePMMA template to produce hollow SnO₂ nanospheres (FIG. 1B and FIG. 4-6).A representative scanning electron microscope (SEM) image in FIG. 1Bshows a SnO₂ nanosphere sample calcined at 500° C. HR-TEM micrographs inFIG. 1C and FIG. 6 indicate the nanosphere walls were constructed fromsmall, interconnected nanocrystals. The lattice fringes of 0.335 and0.264 nm in FIG. 1C correspond to (110) and (101) planes ofpolycrystalline rutile SnO₂.

The PMMA template fixed the nanosphere diameter at 205-210 nm for allcalcination temperatures, and XRD and EXAFS confirmed a consistent SnO₂oxidation state and tetragonal rutile structure. Higher calcinationtemperatures produced sharper, more intense XRD peaks that indicateincreased crystallinity and larger mean crystallite size, and FIG. 1D(circles) demonstrates that the SnO₂ crystallite size scaled withcalcination temperature. These characterizations reveal that both thesize and crystallinity of the constituent SnO₂ nanocrystals werewell-controlled with post-treatment calcination temperature, but wefound calcining at 600° C. produced nanosphere structures with severelyfractured walls (FIG. 4).

Particle size of primary nanoparticles can be measured by electronmicroscopy techniques. Since the inventive particles are spherical, alldiameters are assumed to be the same, but in the general case, the sizeis the minimum diameter through the center.

Electrochemical reduction of CO₂ was conducted at room temperature in anaqueous electrolyte of 0.1M KHCO₃. Typical experiments involved holdingan electrochemical potential for a set amount of time in a gas-tightreactor cell. After a pre-determined amount of time the gaseous reactionproducts were measured with gas chromatography and liquid formateproduction was measured with ion chromatography.

A catalytic figure of merit is defined as the partial current densityfor formate production (j_(formate)/mA cm⁻²). This value describes theamount of electrochemical current per geometric electrode areaassociated with formate production (FIG. 2). In the tested example,formate was produced at rates (partial current densities) approximatelysix times higher than commercially-available materials. Our formateproduction rates are also approximately two times higher than the bestreported materials in scientific literature. Initial stability testingover several hours shows extremely stable performance and consistentproduct formation rates. Importantly, no other liquid products wereformed. The only other byproducts were gaseous CO and H₂ (syngas), whichcould be easily removed from the reactor and for other industrialapplications (methanol synthesis, etc.).

Examples Synthesis of Poly (Methyl Methacrylate) (PMMA) Latex

All chemicals were purchased from Sigma-Aldrich and used as receivedwithout further purification. PMMA latex was prepared by surfactant-freeemulsion polymerization using a cationic free radical initiator. 875 mLof deionized water (DIW) and 100 g of methyl methacrylate(CH₂═C(CH₃)COOCH₃) were mixed at room temperature under a nitrogen flowfor 30 min and then maintained at 70° C. Subsequently, a solutioncontaining 0.15 g of 2,2′-azobis (2-methylpropionamidine)dihydrochloride ([═NC(CH₃)₂C(═NH)NH₂]₂.2HCl) and 25 mL of DIW wasquickly added under vigorous stirring to form a milky white suspension.The suspension was then stirred at 70° C. for 6 h to complete thepolymerization. After cooling down to room temperature for 1 h, theconcentration of obtained PMMA latex (size of ca. 220 nm) was 10 wt %.The latex was diluted with DIW to achieve 0.5 wt % for further use.

Synthesis of Hierarchical Hollow SnO₂ Spheres

All chemicals were purchased from Sigma-Aldrich and used as receivedwithout further purification. Hierarchical hollow SnO₂ spheres weresynthesized by a combined sol-gel and templating method. Poly (methylmethacrylate) (PMMA) spherical template (diameters of ca. 210 nm) wasprepared by surfactant-free emulsion polymerization using a cationicfree radical initiator. In a typical procedure, 226 mg of tin (II)chloride dihydrate (SnCl₂.2H₂O) were dissolved in 5 mL of ethanol(C₂H₅OH, 200 proof) and 38 mg of anhydrous citric acid (C₆H₈O₇) wereseparately mixed in 5 mL of ethanol. Citric solution was then added intotin precursor and sonicated for 15 min. 1.5 mL of tin-citric solute ionwas dropwise added into 30 mL of aqueous PMMA latex template (0.5 wt %)under vigorous stirring at room temperature. After 30 min, the mixturewas evaporated overnight in the oven at 60° C. to obtain theas-synthesized powders. Same stock tin-citric solution was used to makemultiple batches of as-synthesized materials which were subsequentlyannealed in static air at 300, 400, 500 and 600° C. for 3 h with rampingrate of 1° C. min⁻¹. The obtained powder was denoted as “SnO₂nanospheres”.

Non-hierarchical SnO₂ nanoparticles were prepared using similar recipes,except using 30 ml of deionized water in lieu of PMMA dispersion. Afterevaporation at 60° C., the products were subsequently calcined in air at500° C. with ramping rate of 1° C. min⁻¹ for 3 h and named“non-templated SnO₂ nps”. Commercial SnO₂ nanopowder with ≤100 nmaverage particle size (Sigma, product number 549657) was also used asreference material and denoted as “com-SnO₂ nps”.

Electrochemical CO₂ Reduction Measurement

Electrochemical experiments were performed in a gas-tight,two-compartment H-cell separated by a Nafion 117 proton exchangemembrane. Each compartment was filled with 60 mL of aqueous 0.1 M KHCO₃electrolyte (99.99%, Sigma-Aldrich) and contained 90 mL headspace. Theultra-pure deionized water with 18.3 MΩ cm⁻¹ resistivity (BarnsteadEASYpure LF) was used in all electrochemical experiments. The catholytewas continuously bubbled with CO₂ (99.999%, Butler gas) at a flow rateof 20 mL min⁻¹ (pH˜6.8) under vigorous stirring during the experiments.The counter and reference electrodes were Pt mesh and Ag/AgCl (saturatedNaCl, BASK)), respectively. The catalyst ink was composed of 2.8 mg ofthe powder catalysts, 0.32 mg Vulcan VC-X72 carbon black, and 40 μL ofNafion® 117 solution binder (Sigma-Aldrich, 5%) in 400 μL of methanol.Working electrodes were fabricated by drop-casting the ink ontoPTFE-coated carbon paper (Toray paper 060, Alfa Aesar) and N₂-dried. Themass loadings were kept at 9.5±0.6 mg_(ink) cm_(geo) ⁻² and 5.4±0.3mg_(SnO2) cm_(geo) ⁻². Cyclic voltammetry (CV) was obtained inCO₂-saturated KHCO₃ in the potential window of +1 V and −1.3 V vs. RHEwith scan rate of 20 mV s⁻¹. All potentials were referenced against thereversible hydrogen electrode (RHE) (unless otherwise specified),typical uncompensated resistances were 40-50Ω, and the uncompensatedohmic loss (Rn) was automatically corrected at 85% (iR-correction) usingthe BioLogic instrument software in all electrochemical experiments.

CO₂ electroreduction tests were performed at room temperature using aSP-300 potentiostat (BioLogic Science Instrument). The fresh catholytewas saturated with CO₂ by continuously purging with CO₂ (20 mL min⁻¹)under vigorous stirring during the experiments. Short-termchronoamperometric experiments were conducted for 20 min at each appliedpotential between −0.6 V and −1.3 V vs. RHE and the products werecollected every 20 min. Long-term chronoamperometric experiments wereconducted over several days at −1.2 V vs. RHE. The testing was run for 5hours per day and the products were collected every hour. After eachcycle, the electrodes were discarded from electrolyte and naturallystored in polystyrene petri dish for next cycle. Fresh aqueous KHCO₃catholyte was used for each cycle. The total and partial currentdensities were normalized to the exposed geometric area (unlessotherwise specified). Each data point is an average of at least threeindependent experiments on different fresh electrodes. The evolved gasproducts were collected in a Tedlar gas-tight bag (Supelco) and thenquantified by PerkinElmer Clarus 600GC equipped with both FID and TCDdetectors, using ShinCarbon ST 80/100 Column and He as a carrier gas.The liquid products collected from the catholytes at intervals of 20 minor 1 h were filtered with PES 0.22 μm filter and determined by DionexICS-5000+ ion chromatography using ED50 conductometric detector, ASRSsuppressor in auto-generation mode, AS11-HC column and KOH eluent with agradient of 0.4-30 mM in 45 min run.

Materials characterization. Scanning electron microscopy (SEM) imagingwas performed on a FEI Quanta 600F microscope operated at 10-20 kVequipped with an energy-dispersive X-ray (EDX) detector. High-resolutiontransmission electron microscopy (HR-TEM) was carried out on a FEI TitanThemis G2 200 Probe Cs Corrected Scanning Transmission ElectronMicroscope operated at an accelerating voltage of 200 kV. The powdersample was suspended in ethanol, drop-casted onto a holey carbonsupported Cu grid, and naturally dried in air. X-ray powder diffraction(XRD) patterns were collected on a PANalytical X'Pert Pro X-raydiffractometer using CuKα radiation (λ=1.5418 Å) at a scan rate of 0.2°min⁻¹. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI5000 VersaProbe III scanning XPS microprobe (Physical Electronics,ULVAC-PHI Inc) using Al Kα (1486.6 eV) radiation source and ahemispherical analyzer. All the binding energies were internallycalibrated to the surface adventitious hydrocarbon feature (C 1s) at284.6 eV.

Synchrotron X-ray diffraction measurements were conducted at beamline17-BM-B (λ=0.24121 Å) of the Advanced Photon Source at Argonne NationalLaboratory. The post-reaction electrodes under the application of −1.2 Vvs. RHE were collected in the H-cell as a function of electrolysis time.Two-dimensional diffraction patterns were collected by a Perkin Elmeramorphous silicon detector, data acquisition was performed with QXRD andthe diffraction ring was integrated using GSAS-II freeware package.

Raman spectroscopy was performed on a LabRam HR-Evolution spectrometer(Horiba Scientific) with a 633 nm laser as an excitation source and 100×working distance objective, and in situ measurements were carried outusing a custom-made electrochemical cell and a 50× long-working-distanceobjective. The composition of catalyst ink was identical to the one usedin CO₂RR H-cell tests with 5 μl, of the catalyst ink drop-casted onto aglassy carbon working electrode. A Pt wire and Ag/AgCl were used ascounter and reference electrodes, and iR-correction was applied in allmeasurements. 5 mL of 0.1 M aqueous KHCO₃ electrolyte was continuouslypurged with CO₂ during the measurements and sequential Raman spectrawere collected under open circuit and at −1.2 V vs. RHE.

Sn K-edge X-ray absorption spectroscopy (XAS) was collected at the 8-ID(ISS) beamline of the National Synchrotron Light Source II (NSLS-II) atBrookhaven National Laboratory using a Passivated Implanted PlanarSilicon detector and Sn foil for energy calibration (29.2 keV). Allsynthesized SnO₂ samples, bulk SnO₂ and bulk SnO powders were loadedinto Kapton capillary and Sn K-edge data were collected in fluorescencemodes and subsequently analyzed using IFEFFIT freeware package.

The XRD patterns of 3D SnO₂ nanospheres calcined from 300 to 600° C.were indexed to pure tetragonal SnO₂ rutile (JCPDS 41-1445) having thespace group P4₂/mnm. Increasing calcination temperature producedsharper, more intense, peaks that indicate increased crystallinity andcrystallite size up to ˜10 nm. In addition, the Sn K-edge EXAFS resultsin show the presence of first nearest neighbor shell of Sn—O and secondSn—Sn coordination shell for all SnO₂ sphere samples. Higher calcinationtemperature led to more intense amplitude of these features, furtherindicating increased crystallinity, particle size, and coordinationnumbers, with less disorder.

The symmetrical Sn 3d_(5/2) and Sn 3d_(3/2) doublet in core-level XPScorresponds to Sn⁴⁺ oxidation state in rutile SnO₂. The SnO₂ nanospheresshowed up-shifted Sn 3d peaks compared with bulk SnO₂, and lowercalcination temperatures (smaller SnO₂ nanocrystals) produced largerbinding energy (BE) increases. Similar size-dependent BE shifts havealso been observed for other small SnO₂ nanoparticles,²⁵ as well asnanoparticulate Au,²⁶ Pd,²⁷ and PbS²⁸ systems. There was no evidence ofSn²⁺ or any tin-related impurity phases using other characterizations,including XRD and Raman results.

XRD results of non-templated SnO₂ nanoparticles and commercialnanoparticles demonstrate tetragonal rutile SnO₂ crystal structure.Non-templated SnO₂ nps had almost identical crystallinity, orientation,crystallite size (˜7 nm) and structural defects as 3D hierarchical SnO₂nanospheres prepared at same temperature (500° C.). However, commercialSnO₂ nanoparticles possessed 4.4 wt % orthorhombic SnO₂ phase (JCPDS78-1063, space group Pbcn), much larger crystal size (ca. 28 nm).Similarly, Sn K-edge EXAFS spectra also showed the first nearestneighbor shell of Sn—O and second Sn—Sn coordination shell for twonanoparticle samples. The Sn 3d doublets also indicated the presence ofSn⁴⁺ valence state in both non-hierarchical nanoparticle samples.

SnO₂ nanospheres show characteristic Raman bands including A_(1g)(symmetric Sn—O stretching), B_(2g) (asymmetric Sn—O stretching), doublydegenerated E_(g) modes (space group D_(4h)), and broad E_(u) and A_(2g)scattering peaks, as previously noted. In situ Raman spectroscopy wasconducted to determine the change in oxidation state during applicationof electrochemical potential relevant to CO₂RR.

In situ time-dependent Raman spectra of SnO₂ nanospheres calcined at500° C. (on glassy carbon electrode) under CO₂RR at −1.2 V vs. RHEshowed that the A_(1g) and smaller E_(g) and E_(u) peaks were stillvisible for the SnO₂ nanosphere catalysts deposited on a glassy carbonelectrode and held at open circuit in CO₂ saturated electrolyte.Time-resolved Raman spectra collected at −1.2V vs. RHE showed theattenuation and then complete disappearance of characteristic Ramanbands. This result is consistent with the time-dependent XRD shown inFIG. 3C and provides further evidence for the reduction of SnO₂ intometallic Sn during CO₂RR. No other peaks associated with reduced tinoxides and/or surface-bound intermediate species were observed in thewide region of 150-850 cm⁻¹. Our observation is consistent with operandoRaman results for reduced graphene oxide supported SnO₂ reported byDutta et al.⁴⁵ Oxide fingerprints completely disappeared at verynegative potentials, particularly −1.55 V vs. Ag/AgCl, as the catalystfully reduced to metallic Sn. The re-emergence of characteristic SnO₂Raman bands when the electrode was held at open circuit afterelectrolysis indicates re-oxidation of the metallic Sn into oxidespecies.

Electrochemical CO₂ reduction performance in an aqueous H-cell. CO₂RRactivity was screened between −0.6 V to −1.3 V vs. RHE in an H-cellcontaining CO₂-saturated 0.1 M KHCO₃. All SnO₂ electrocatalysts producedformate as a main product, along with smaller amounts of CO and H₂ (FIG.7), but SnO₂ spheres calcined at 500° C. exhibited the highest FE andformate partial current density (j_(formate)) at all potentials (FIG. 2Aand FIG. 7-9). This 500° C. SnO₂ nanosphere catalyst contained −7 nmprimary nanocrystals, and FIG. 2B shows that it produced 71-81% formateFE between −0.9 V and −1.3 V vs. RHE and a maximum j_(formate) of 73±2mA cm_(geo) ⁻², which are among the highest performance metrics reportedfor Sn-based electrocatalysts in aqueous H-Cells (Table 1). The FEs forC1 products reached >90% in the range from −0.8 V to −1.2 V and the H₂evolution reaction was strongly suppressed. It is worth mentioning thatgaseous CO and H₂ side-products (syngas) are easily separated fromliquid formate for subsequent use in methanol or Fischer-Tropschsynthesis.

The results in FIG. 2A also show an apparent dependence on the size ofthe constituent SnO₂ nanocrystals. It has been reported previously thatgrain boundaries,^(4,12,13,18,35,38,39-41) oxygen vacancies,^(37,42,43)and particle size^(31,35,37,43) of SnO₂ can impact CO₂RR activity andselectivity. In this study, we suggest that SnO₂ nanospheres annealed at500° C. likely produced an optimum balance between crystallinity andnanocrystal size that maximized formate selectivity and production rate.

We also compared the performance of SnO₂ nanospheres with similar sized(˜7 nm), non-templated SnO₂ nps and commercially available SnO₂ nps(named com-SnO₂ nps) with a heterogeneous particle size distributionbetween 5-150 nm (FIG. 10). Non-templated SnO₂ nps were synthesized withan identical procedure except without the polymer template, and thencalcined at 500° C. FIG. 2C and FIG. 11 show the SnO₂ nanospheresdemonstrated a 2˜6-fold improvement in formate partial current density,20-30% higher formate FE, and reduced H₂ evolution compared with thenon-templated and commercial SnO₂ nps. Capacitance-based electrochemicalsurface area (ECSA) measurements^(9,18,19,38) indicated the SnO₂nanospheres demonstrated approximately 1.5-3 times larger ECSA than thenon-templated and commercial SnO₂ nps (FIG. 12 and Table 2), but allthree samples produced comparable ECSA-normalized formate partialcurrent density (FIG. 2D). This result indicates the total amount ofelectrochemically active surface area was the dominant influence ongeometric formate partial current density. In this regard, controllingthe SnO₂ nanosphere surface structure improved geometric-basedperformance over commercially available and non-templated SnO₂ nps bymaximizing ECSA.

TABLE 2 Double-layer capacitance (C_(dl)) and electrochemical surfacearea (ECSA) for SnO₂ nanospheres calcined at 500° C., non-templated SnO₂nps, and com-SnO₂ nps. All measurements were carried out in CO₂-purged0.1M KHCO3 and all electrodes had equivalent SnO₂ loadings of 5.4 ± 0.3mg_(SnO2) cm_(geo) ⁻² (total ink loading, including SnO₂ and carbonblack, was 9.5 ± 0.6 mg_(ink) cm_(geo) ⁻²). Sample C_(dl) [mF cm⁻²] ECSA[cm²] SnO₂ nanospheres 14.52 51.3 Non-templated SnO₂ nps 3.24 31.8Com-SnO₂ nps 2.67 16.8

The long-term durability of SnO₂ nanospheres, non-templated SnO₂ nps,and commercial SnO₂ nps was evaluated in an H-Cell at −1.2 V vs. RHEwith multiple start/stop cycles. As seen in FIG. 3A, formate partialcurrent density for the SnO₂ nanosphere catalysts stabilized at anaverage 45±5 mA cm_(geo) ⁻² over 36 hours of operation with an average68±8% FE. Non-templated SnO₂ nps and com-SnO₂ nps produced a smaller −20mA cm_(geo) ⁻² and similar ˜70% FE during steady state operation.Post-electrolysis SEM imaging in FIG. 3B revealed severe particleagglomeration or coalescence for the non-templated SnO₂ nps andcommercial SnO₂ nps after 20 hours of electrolysis at −1.2V vs. RHE.This behavior has been observed before and agglomeration is a knowndeactivation mechanism for SnO₂ electrocatalyts.^(18,44) In contrast, nosubstantial particle agglomeration was observed for the SnO₂nanospheres, which may stem from the interconnected SnO₂ nanocrystalswithin the nanosphere walls preventing severe particle growth underthese conditions. No evidence of trace contaminant deposition on theelectrode surface, such as Pt, Fe, Pb, or Zn, was detected on theelectrode surface after long-term electrolysis (FIG. 13).

Time-dependent, synchrotron-based XRD of SnO₂ nanospheres operated at−1.2 V vs. RHE revealed the reduction of SnO₂ nanocrystals into metallicSn through the emergence of body-centered tetragonal β-Sn diffractionpeaks (FIG. 3C). These results indicate rapid transformation of SnO₂into metallic Sn and a slight increase in crystallite size to 23-24 nmunder steady state operation (FIG. 14). Notably, this crystallite sizeremained stable over 30 h of operation and the XRD data agrees well withpost-reaction SEM imaging that ruled out severe particle growth duringlong-term electrolysis. We also observed a minor residual oxide phasethat likely resulted from re-oxidation upon air exposure. These resultsstrongly support complementary in situ Raman spectroscopy experimentsthat showed SnO₂ was reduced to metallic Sn during CO₂RR at −1.2V, whichis consistent with previous operando Raman results for other SnO₂ CO₂RRelectrocatalysts.⁴⁵

Calculation of Faradaic efficiency and selectivity

-   -   The Faradaic efficiency (FE) for product i is defined as the        percentage of supplied electrons used to convert CO₂ into        product i and calculated as follows:

${FE}_{i} = {\frac{z_{i}*F*n_{i}}{I*t} = \frac{z_{i}*F*n_{i}}{Q}}$

where z_(i) is the number of electrons involved in the formation ofproduct i (z=2 for formate, CO, and H₂); F is the Faraday's constant(96485 C mol_(e) ⁻¹); n_(i) is the number of moles of product i formed(determined by GC and IC); I is the total current; t is electrolysistime; and Q is total charge in Coulombs passed across the electrode.

-   -   The formate selectivity is defined as molar ratio of formate        compared with the total CO₂RR products:

$S_{formate} = \frac{{mol}_{formate}}{{mol}_{formate} + {mol}_{CO}}$${{or}:S_{formate}} = \frac{2*r_{formate}}{{2*r_{formate}} + {2*r_{CO}}}$

where r is production rate for a reduced product, and 2 is the number ofelectrons involved in the formation of CO and HCOOH.

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TABLE 1 Comparison of CO₂-to-formate performance of 3D hierarchical SnO₂spheres and other Sn based electrocatalysts reported for formicacid/formate production (excludes mixed metal oxides, alloys, and dopedsystems). HCOO− current Operating Faradaic density Catalysts Electrolytepotential [V] efficiency [%] [mA cm ²] References 0.1M KHCO₃ −1.2 V vs.RHE 81.1 50.4 This work 3D SnO₂ nanospheres 0.1M KHCO₃ −1.0 V vs. RHE76.6 28.9 This work Hierarchical Sn dendrite 0.1M KHCO₃ −1.36 V vs. RHE71.6 12.2 Won et al., ChemSusChem 2015, 8, 3092 Nanoporous Sn foam 0.1MNaHCO₃ −2.0 V vs. Ag/AgCl 90 20.7 Du et al., ChemistrySelect 2016, 1,1711 Chainlike mesoporous SnO₂ 0.1M KHCO₃ −1.06 vs. RHE 82 13.5 Bejtkaet al., ACS Appl. Energy Mater. 2019, 2, 3081 SnO₂ nanoparticles 0.1MKHCO₃ −1.1 V vs. RHE 85 20.1 Daiyan et al., Adv. Sci. 2019, 6, 1900678Grain boundary rich 1M KOH −0.73 V vs. RHE 74 51.8 Liang et al., J.Mater. Chem. SnO₂ nanoparticles (<5 nm) A 2018, 6, 10313 Ultrathin sub-2nm 0.1M KHCO₃ −1.156 V vs. RHE 87.3 13.7 Liu et al., Angew. Chem. Int.SnO₂ quantum wires Ed. 2019, 58, 8499 Nanoporous SnO₂ 0.1M KHCO₃ −1.2 Vvs. RHE 90 17.1 Liu et al., GreenChE. 2022, in press Ultra-small SnO0.5M KHCO₃ −0.9 V vs. RHE 66 13.2 Gu et al., Angew. Chem. Int.nanoparticles/carbon black Ed. 2018, 57, 2943 SnO₂-carbon nanotubes 0.5MKHCO₃ −0.77 V vs. RHE 76 4.6 Pavithra et al., Catal. Sci. Technol. 2020,10, 1311 Hierarchical SnO₂ 0.5M NaHCO₃ −1.6 V vs. Ag/AgCl 87 45 Li etal., Angew. Chem. Int. nanosheets/carbon cloth Ed. 2017, 56, 505 SnO₂0.5M KHCO₃ −0.7 V vs. RHE 90 40 Zhang et al., Chem. Eng. J.nanosheets/N-doped carbon cloth 2021, 421, 130003

1. A SnO₂ powder, comprising at least 90 mass % hollow spheres in the(diameter) size range of 175 to 225 nm, preferably 180 to 220 nm, insome embodiments 190 to 210 nm; and wherein the spheres are comprised ofSnO₂ particles.
 2. The SnO₂ powder of claim 1 wherein at least 90 mass %hollow spheres are in the (diameter) size range of 180 to 220 nm or 190to 210 nm
 3. A SnO₂ powder, comprising hollow spheres having a diameterof 100 nm or greater, wherein the spheres are comprised of SnO₂particles, and wherein at least 90 mass % of the spheres have diameterswithin a 10 nm range.
 4. The SnO₂ powder of claim 3 wherein at least 90mass % of the spheres have diameters within a size range of from 200 to220 nm.
 5. The SnO₂ powder of claim 1 wherein at least 90 mass % of thespheres have diameters within a 7 nm size range or a 5 nm size range ora 3 nm size range.
 6. The SnO₂ powder of claim 1 wherein the hollowspheres have a wall thickness in the range of 20 to 35 nm or 25 to 30nm.
 7. The SnO₂ powder of any of claim 3 wherein the hollow spheres arecomprised of nanocrystals having a mass average diameter in the range of5 to 15 nm, or 5-10 nm, or 6 to 9 nm.
 8. The SnO₂ powder of claim 1wherein, as measured by XRD, the hollow spheres are comprised ofnanocrystals having an average crystallite size in the range of 5 to 10nm, or 6 to 9 nm, or 6 to 8 nm.
 9. The SnO₂ powder of claim 3characterizable by a durability of maintaining a j_(formate) (mA cm⁻²)of at least 35 at 1.2 V vs. RHE for at least two days withoutregeneration.
 10. (canceled)
 11. The SnO₂ powder of claim 1characterizable by a j_(total)/mA cm⁻² _(geo) of at least 50 at 1.2 Vvs. RHE.
 12. (canceled)
 13. The SnO₂ powder of claim 3 characterizableby an ESCA of at least 35 cm².
 14. A method of making a SnO₂ catalyst,comprising: providing a suspension of polymer particles, combining a tinsalt with the suspension, removing the liquid from the suspension(preferably by evaporation) to form tin-coated polymer particles, dryingthe tin-coated polymer particles, and calcining the dried particles toburn out the polymer particles leaving hollow SnO₂ spheres.
 15. Themethod of claim 14 wherein the suspension is an aqueous suspension. 16.(canceled)
 17. The method of claim 14 wherein the calcining is carriedout at a temperature in the range of 300 to 600° C.
 18. A catalyst ink,comprising the SnO₂ particles powder of claim 1 dispersed in a liquidphase along with conductive particles and binder particles. 19-20.(canceled)
 21. The catalyst ink of claim 18 wherein the conductiveparticles comprise carbon black, carbon fibers, carbon or graphenesheets, or carbon nanotubes. 22-24. (canceled)
 25. The catalyst ink ofclaim 18 wherein the liquid phase comprises at least 50% of an alcoholor mixture of alcohols.
 26. An electrode, comprising a conductivesubstrate coated with the SnO₂ powder of claim
 1. 27. The electrode ofclaim 26 wherein the conductive substrate comprises a porous carbonpaper.
 28. A method of making an electrode by impregnating, drop-castingor coating the ink of claim 18 into or on a conductive substate.
 29. Asystem comprising the electrode of claim 26 disposed in a solution thatis saturated with CO₂
 30. A system comprising the electrode of claim 26comprising a tin catalyst disposed in a solution that is saturated withCO₂, and further wherein the system or catalyst is characterizable by adurability of maintaining a j_(formate) (mA cm²) of at least 35 or atleast 40 or in the range of 40 to 55 at 1.2 V vs. RHE for at least oneor at least two or at least three days or from one to five days. 31-37.(canceled)